The present invention relates to the digital image processing arts. In particular, it relates to enhancement of digital image data, and a method and apparatus for attenuating overshoot in the luminance channel based upon the chrominance channel when edge enhancement operations are performed. Thus, the present invention provides a method and apparatus for reducing color shift and ringing due to edge enhancement operations without compromising resulting edge sharpness.
A main advantage of digital image reproduction relative to traditional light-lens image reproduction resides in the ability to process the digital image data so as to enhance the appearance of the final output image. For example, digital filtering is often performed to sharpen edges and fine lines for purposes of making an output image more visually appealing. When reducing or scaling-down an image, pre-filtering is often performed prior to sub-sampling for purposes of anti-aliasing. Due to limitations of the filtering algorithm, this filtering operation can blur an image. Also, in color image reproduction, it is a common practice to sub-sample the chrominance information to reduce the number of bits necessary to represent an image. This, too, can result in blurred edges. In either case and others, edge enhancement of the filtered image can be performed to sharpen the final output image.
One drawback associated with conventional edge enhancement operations is that the luminance information can be altered significantly from its original value so that the color of the final output image is shifted compared to the original image. Another undesirable effect of enhancement operations is the formation of dark or light rings about colored image regions. Color fringing and ringing are, obviously, undesirable and must be controlled in order to provide an aesthetically pleasing final image. On the other hand, luminance channel overshoot is less important to the appearance of the final output image if it occurs in areas of the image that are less colorful or xe2x80x9cneutral,xe2x80x9d e.g., in regions of black-and-white text data. Heretofore, no effective method and apparatus have been provided for controlling luminance channel overshoot due to image enhancement operations without compromising edge sharpness. Furthermore, no effective method and apparatus for controlling luminance channel overshoot have been provided wherein the overshoot attenuation is varied depending upon the chrominance of the image data.
In accordance with the present invention, a new and improved method and apparatus are provided for luminance channel overshoot control in image enhancement operations.
In accordance with a first aspect of the present invention, a method of digital image processing comprises receiving input digital image data defining a plurality of pixels of an input color digital image, wherein the input digital image data includes at least a luminance value and a chrominance value for each of the pixels of the input image. For each of the pixels of the input digital image, the input digital image data defining the pixel is enhanced based upon the input digital image data defining neighborhood pixels in a spatial neighborhood established about the pixel to obtain enhanced digital image data defining the pixel, the enhanced digital image data including at least an enhanced luminance value. For each of the pixels of the input digital image, an overshoot control operation is performed on the enhanced digital image data defining the pixel, wherein the overshoot control operation comprises: (i) determining local maximum and local minimum input luminance values in a spatial neighborhood about the pixel; (ii) comparing the enhanced luminance value of the pixel to the local maximum and local minimum luminance values to determine if the enhanced pixel luminance value is: (a) above the local maximum luminance value by a positive overshoot amount; or, (b) below the local minimum luminance value by a negative overshoot amount; (iii) receiving an input overshoot reduction coefficient; (iv) evaluating the chrominance of the pixel; (v) using the chrominance of the pixel to adjust the input overshoot reduction coefficient so as to obtain a modified overshoot reduction coefficient; (vi) if the enhanced luminance value of the pixel is above the local maximum luminance value, reducing the enhanced luminance value to attenuate the positive overshoot amount by a quantity that depends upon the modified overshoot reduction coefficient; and, (vii) if the enhanced luminance value of the pixel is below the local minimum luminance value, increasing the enhanced luminance value to attenuate the negative overshoot amount by a quantity that depends upon the modified overshoot reduction coefficient.
In accordance with another aspect of the present invention, a method of enhancing a color digital image comprises, for each of a plurality of pixels defining the color digital image, modifying a luminance value of a subject pixel in response to an enhancement filtering operation to obtain a modified luminance value for the subject pixel. Within a neighborhood of pixels spatially near the subject pixel, identifying a local minimum pixel luminance value and a local maximum pixel luminance value. The chroma of the subject pixel is determined and an overshoot reduction coefficient is received. The overshoot reduction coefficient is modified based upon the chroma of the subject pixel to obtain a modified overshoot reduction coefficient, and the modified luminance value is compared to at least one of the local minimum and local maximum luminance values. If the modified luminance value is greater than the local maximum luminance value, the modified overshoot reduction coefficient is used to reduce the modified luminance value of the subject pixel by a percentage of the amount by which the modified luminance value exceeds the local maximum luminance value. If the modified luminance value is less than the local minimum luminance value, the modified overshot reduction coefficient is used to increase the modified luminance value of the subject pixel by a percentage of the amount by which the modified luminance value is less than the local minimum luminance value.
In accordance with still another aspect of the present invention, a digital image processing apparatus comprises means for receiving input digital image data defining a plurality of pixels of an input color digital image, the input digital image data including at least a luminance value and a chrominance value for each of the pixels of the input image. A means adapted for enhancing each of the pixels of the input digital image based upon the input digital image data defining neighborhood pixels in a spatial neighborhood established about each of the pixels to obtain enhanced digital image data defining each of the enhanced pixels is provided, the enhanced digital image data including at least an enhanced luminance value. A means is also provided for performing an overshoot control operation on the enhanced digital image data defining each enhanced pixel, wherein the means for performing an overshoot control operation includes: (i)means for determining local maximum and local minimum input luminance values in a spatial neighborhood about each enhanced pixel; (ii) means for comparing the enhanced luminance value of each enhanced pixel to the local maximum and local minimum luminance values to determine if the enhanced pixel luminance value is: (a) above the local maximum luminance value by a positive overshoot amount; or, (b) below the local minimum luminance value by a negative overshoot amount; (iii) means for receiving an input overshoot reduction coefficient; (iv) means for evaluating the chrominance of the enhanced pixel as defined by the chrominance value; (v) means for using the chrominance of the enhanced pixel to adjust the input overshoot reduction coefficient so as to obtain a modified overshoot reduction coefficient; (vi) means for reducing the enhanced luminance value to attenuate the positive overshoot amount by a quantity that depends upon the modified overshoot reduction coefficient if the enhanced luminance value of the pixel is above the local maximum luminance value; and, (vii) means for increasing the enhanced luminance value to attenuate the negative overshoot amount by a quantity that depends upon the modified overshoot reduction coefficient if the enhanced luminance value of the pixel is below the local minimum luminance value.
In accordance with yet another aspect of the present invention, a method of processing a digital image includes enhancing a luminance value (L*) of each of a plurality of input pixels (P) to obtain an enhanced luminance value (FIR_OUTPUT) for each pixel. A positive overshoot is associated with a subject pixel of said digital image is identified if the enhanced luminance value (FIR_OUTPUT) of the subject pixel is greater than a local maximum luminance value (LOCAL_MAX) of input pixels (P) in a spatial neighborhood of the subject pixel in the digital image by a positive overshoot amount (OS). A negative overshoot associated with a subject pixel of the digital image is identified if the enhanced luminance value (FIR_OUTPUT) of the subject pixel is less than a local minimum luminance value (LOCAL_MIN) of input pixels (P) in a spatial neighborhood of the subject pixel in the digital image by a negative overshoot amount (OSxe2x80x2). An overshoot reduction coefficient (f) is provided for controlling the amount by which positive or negative overshoot associated with the subject pixel is attenuated. The overshoot reduction coefficient (f) is adjusted according to a chrominance of the subject pixel so that the overshoot reduction coefficient (f) is adjusted to have increasing overshoot attenuation effect as the chrominance of the subject pixel increases. The overshoot reduction coefficient (f), as adjusted based upon the chrominance of the subject pixel, is used to attenuate either the positive or negative overshoot associated with the subject pixel.
One advantage of the present invention is the provision of a method and apparatus for luminance channel overshoot control during digital image processing enhancement operations.
Another advantage of the present invention resides in the provision of a method and apparatus for controlling luminance channel overshoot resulting from edge enhancement processing wherein color shift and ringing are reduced or minimized without compromising edge sharpness.
A further advantage of the present invention is found in the provision of a method and apparatus for reducing luminance channel overshoot using original min/max luminance values from pixels in the neighborhood surrounding a pixel being processed and based upon the chrominance channel values for the pixel being processed.
A yet further advantage of the present invention resides in the provision of a method and apparatus for attenuating luminance channel overshoot in edge enhancement operations, wherein the attenuation varies depending upon whether the overshoot is positive or negative.
Still other benefits and advantages of the present invention will become apparent to those of ordinary skill of the art to which the invention pertains upon reading and understanding the present specification.